Looking Up: Watch out for the next supernova
Stars are not as steady as they first appear. Constellations of stars remain our faithful friends, returning night after night or year to year. For the most part the stars never seem to change.
Many stars are known to vary periodically in brightness, and it is interesting to track them and plot their estimated magnitude on a chart, with the magnitude scale on the vertical side and he dates or even hours on the horizontal line or bottom of your chart.
You end up with a graph showing how the star peaked in brightness and slowly curved downward toward minimum light.
There are many classifications of variable stars. Some change in brightness because they have a dimmer star revolving around it, periodically eclipsing. Others have within their nature something that causes the star to pulsate and vary in output.
Resources such as Sky and Telescope Magazine and Astronomy Magazine alert you to some of these. They have finder charts which label the brightness of stars nearby by which to make your comparison estimations of the variable star.
None of these, however, are as startling as a supernova.
These are extremely rare events where a star literally explodes in a fury unmatched by another star you have likely ever seen. On a few occasions in history, mankind has marveled in wonder and even fear at the sight of a suddenly brilliant star where no star had been known to exist, outshining all the stars in the sky and even rivaling the moon.
An international team of astronomers recently announced observation of an unusually large supernova, explosion of a star as big as 150 to 240 solar masses. A normal supernova is a star no bigger than about 100 times the sun’s mass.
Just over a thousand years ago, a star blew up and shocked the world with a brightness some 15 times greater than brilliant Venus at its best. At magnitude -7.5, people actually saw it in broad daylight. Even as it faded, it remained visible at night for more than two years- in those days long before the invention of the telescope. The star appeared in the constellation Lupus the Wolf, visible on May evenings low in the southeast. It was best seen from the southern hemisphere where Lupus is high in the sky.
Astronomers tell us that the supernova likely was a “type I-a”- a white dwarf star that gathered too much stellar material from a companion star, and detonated.
Other brilliant supernovae occurred in 1054 in Taurus the Bull; 1572 in Cassiopeia; and 1610 in Scorpius.
No one knows when the next one will occur, but the Milky Way Galaxy is long overdue. Astronomers have observed many of them in other galaxies and can estimate how often they happen. A supernova becomes the brightest star in the entire galaxy and flashes its light across the universe.
They are expected to occur on average once a century, but the last one known in the Milky Way was more than 400 years ago. If one occurs tonight, some might say it is a sign of the times, a portender of the end of the age, but it is best to remember they have occurred before, and any such “sign” must be taken in context with other events if it is one at all.
After a supernova has faded away, they leave behind an expanding, nebulous cloud visible in telescopes. The Crab Nebula in Taurus is a familiar backyard telescope target and is what is left from the 1054 explosion.
Cassiopeia the vain queen
Prominent high in the northern sky on a December evening is the constellation Cassiopeia, its five principal stars forming a large “M.” The folklore behind Cassiopeia the Queen was handed down from the Greeks about 3,500 years ago. Boasting she was more beautiful than the sea nymphs, the nymphs complained to Neptune. To punish her, Neptune decreed that her daughter Andromeda be chained to a rock at the seashore and serve as fish bait.
The unfortunate maiden was rescued by her boyfriend, Perseus - a son of Jupiter - who arrived with the hideous one-eyed head of Medusa. Holding this up to the monster’s eyes caused the sea monster to turn to stone. Perseus then carried Andromeda away. Mercury had given Perseus the ability to fly, by putting on his feet winged sandals. Perseus was awarded with Andromeda’s hand in marraige.
Today Cassiopeia, her husband King Cepheus, Andromeda and Perseus are immortalized by being honored with constellations in the same part of the sky. Neptune of course is the name of the eighth planet which was unknown to the ancient Greeks. Fortunately we guess, Neptune’s orbit never goes near these constellations!
Last-quarter moon is on Dec. 8.
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Keep looking up!